Fluid pressure control valves and braking apparatus

ABSTRACT

A fluid pressure control valve having a restricted flow inlet, an outlet and an exhaust port provided with a valve seat. A throttling element is movable towards and away from the valve seat to vary the throttling effect and thereby vary the pressure at the outlet port in proportion to the amount of throttling. A cantilever-mounted leaf-spring has its free end operatively engaging the throttle element. Loading means, operable in response to a predetermined signal, controls movement of the free end of the leaf-spring and hence movement of the throttling element. The exhaust port is carried by a member which is movable towards and away from the throttling element to effect adjustment to the minimum throttling effected by the throttling element.

United States Patent [72] Inventors Charles F. B. Shattock;

Oswald G. Shanks; Sydney A. Stevens; Boguslaw W. Wojteeki, London,England;

Robert J. Dixon, Palos Verdes Peninsula,

Primary Examiner-Duane A. Reger Atlorney- Larson, Taylor & HindsABSTRACT: A fluid pressure control valve having a restricted flow inlet,an outlet and an exhaust port provided with a valve seat. A throttlingelement is movable towards and away from the valve seat to vary thethrottling effect and thereby vary the pressure at the outlet port inproportion to the amount of throttling. A cantilever-mounted leaf-springhas its free end operatively engaging the throttle element. Loadingmeans, operable in response to a predetermined signal, controls movementof the free end of the leaf-spring and hence move ment of the throttlingelement. The exhaust port is carried by a member which is movabletowards and away from the throttling element to effect adjustment to theminimum throttling effected by the throttling element.

PATENTEDncI SIS?! 3.610.704

sum 1 [1F 3 'PATENTEDUBT 5m! 3.610104 SHEET 3 0F 3 Fig. .3.

FLUID PRESSURE CONTROL VALVES AND BRAKING APPARATUS This application isa division of our plication Ser. No. 600,799, filed Dec. No. 3,503,656issued Mar. 31, 1970.

This invention relates to fluid-pressure control valves.

The present invention provides a fluid-pressure control valve having arestricted fluid flow inlet port, an outlet port, an exhaust portproviding a valve seat, a throttling element movable towards and awayfrom the valve seat thereby to effect a varying degree of throttling tothe passage of fluid through the valve seat, a cantilever-mountedleaf-spring the free end of which is operatively engageable by thethrottling element, and loading means by which the free-end of theleaf-spring can be loaded such that, in operation of the valve, when afluid pressure of predetennined value pertains at the inlet port, thevalue of the fluid-pressure pertaining at the outlet port is dependentupon the rate of escape of fluid through the valve seat which rate is inturn determined by the degree of throttling effected by the throttlingelement consequent upon the degree of loading of the leaf-spring by saidmeans.

The exhaust port may be carried by a member which is movable towards andaway from the free end of the leaf-spring to effect adjustment of theminimum throttling effected by the throttling element. The member mayhave therein a first conduit one end of which provides the valve seat,and transverse second conduits each communicating with the firstconduit, one of the second conduits being in communication with theinlet port and the other of the second conduits being in communicationwith the outlet port, the member being axially movable within a bore ina body of the valve to efiect said adjustment and the member beingsealed with respect to the bore such as to prevent intercommunication ofthe three ports except by way of the first conduit. The arrangement maybe such that thethrottling element is urged by its weight in a directiontending to seat the element on the valve seat. The throttling elementmay conveniently be constituted by a ball. In this case, the leaf-springmay be provided adjacent its freeend with a cup-shaped button whichserves loosely to locate the element in position.

The loading means may be electromagnetically operable. In this case, theloading means may include a permanent magnet and a cooperating armaturecoil. The permanent magnet may be fixedly located and the electromagnetmay be carried by the leaf-spring adjacent the free end thereof. In thisarrangement, the electromagnet may be annular and may then be mountedcoaxially with respect to the button when provided.

The inlet port may include a choke.

The inlet port may be arranged to be supplied with fluid under pressurevia a pressure-limiting valve.

The outlet port may be connected to a relay valve the operation of whichis controlled by the pressure pertaining at the outlet port. If thefluid-pressure control valve is used in a context in which for most ofthe time the value of the fluidpressure pertaining at the outlet port isrequired to be substantially at atmospheric pressure, this will beachieved by having a minimum throttling effect at the valve seat suchthat there is thereby a continuous loss of fluid through the valve seatof so long as the fluid-pressure at the outlet port is substantially atatmospheric pressure. Such loss of fluid through the valve seat maybeobjectionable and in order to overcome this there may be provided in thefluid path to the inlet port, a further valve by which supply of fluidthrough the inlet port can be prevented.

The operation of the further valve may be so connected with the movementof the throttling element, that immediately upon movement of thethrottling element towards the valve seat the further valve is operatedto allow fluid pressure to pertain at the inlet port.

Where the means of the fluid-pressure control valve iselectromagnetically operable, said further valve may also beelectromagnetically operable. In this case, upon operation of theloading means the further valve may also be immediately operated toallow pressure to pertain at the inlet port.

previous copending ap- 12, 1966 now US. Pat.

There may also be provided a cutoff means by which the supply of fluidto the valve seat is prevented in the event of the pressure of the fluidwhich would otherwise be supplied to the valve seat being below apredetermined value.

Where as above-described, the exhaust port is carried by a membermovable towards and awayfrom the free end of the leaf-spring to effectadjustment of the minimum throttling by the throttling element, thecutoff means may conveniently be embodied in the member. In such a case,the member may have therein a passage leading to the valve seat, passageclosure means by which the passage may be sealed, and pressuresensitivemeans operatively connected to the passage-closure means and subjectableto the pressure of the fluid supplied to the valve seat, thepressure-sensitive member being resiliently loaded in a direction toefiect closure of the passage by the passage-closure means.

There may be provided supplementary loading means by which,independently of the first mentioned loading means, the free-end of theleaf-spring can be loaded in a similar manner to achieve 'a similarresult in loading thereof by the first mentioned means achieved. Thesupplementary loading means may be resiliently biased to effectresilient loading of the leaf-spring and the supplementary loading meansincludes overrising means by which the resilient bias is overcomenormally to prevent the bias being effective. The overriding means maybe variably operative so as, at will, to be operative to a varyingdegree to counteract the resilient bias, and may comprise anelectromagnetic device which is energized to overcome the resilientbias.

Embodiments of the present invention will now be described in greaterdetail, by way of example only with reference to the accompanyingdrawings, of which:

FIG. 1 shows a cross-sectional view of a fluid pressure control valve,

FIG. 2 shows a cross-sectional view of the relevant parts of amodification of the valve shown in FIG. 1,

FIG. 3 shows a cross-sectional view of a further modification of thevalve of FIG. 1,

Referring to FIG. I of the drawings, the control valve comprises a valvebody 1 secured to the underside of a mounting plate 2.

The valve body 1 is provided with a stepped bore 3 within which isaxially adjustable a member 4 having therein an axially extending firstconduit 5 sealed at its lower end 6 and providing at its upper end anexhaust port 7 constituting a valve seat. Extending transversely of theconduit 5 are two second conduits 8 and 9 respectively. The conduit 8communicates with an outlet port 10 and the conduit 9 communicates withan inlet port 11. The member 4 is sealed with respect to the bore 3 bysealing means 12 efiective to prevent intercommunication of the parts 7,l0 and 11 except by way of the first conduit 5.

Seated by its own weight on the exhaust element 13 in the form of aball.

The ball 13 is engageable with the free end of a cantilevermountedleaf-spring 14, through an inversely mounted cupshaped button 15 whichserves loosely to locate the element 13 and which is carried adjacentthe free end of the leaf-spring 14 which is clamped at its other endbetween mounting blocks 16 secured to the mounting plate 2.

Coaxially mounted with respect to the button 15 and carport 7 is athrottling ried by the leaf-spring 14 adjacent its free end thereof, is'a lightweight former l7 encircled by an annular armature coil 18 lyingbetween the inner pole l9 and the encircling outer pole 20 of apermanent magnet 21 fixedly located on an enclosing housing 22 securedto the mounting plate 2 by pillars (of which one only is shown) 23.

The inlet port 11 communicates through a choke 25 with an outlet port 26of a pressure limiting valve 27. The pressure limiting valve 27comprises a valve seat 28 into and out of engagement with which ismovable a ball-shaped valve closure element 29 lying between the outletport 26 and an inlet port 30. The valve closure element 29 is arrangedfor operation by a push rod 31 carried by a diaphragm 32 the undersideof which is resiliently loaded by a spring 33 and to the top side ofwhich is applied (through a passage 34 encircling the push rod 31) thepressure pertaining at the outlet port 26. The outlet port 26 is alsoconnected through a conduit 35, with a volume 36. The inlet port 30 isconnected through a passage 40, to a source of supply of fluid underpressure. The above-described control valve operates as follows:

With the passage 40 connected to a source of fluid under pressure, fluidpressure will build up in the chamber 36 and at the outlet port 26 ofthe limiting valve 27 until this pressure reaches the limit determinedby the setting of the valve 27. At this limit, the downward force on thediaphragm 32 exerted by the pressure of the fluid above the diaphragm32, will balance the force exerted upwardly on the diaphragm 32 by thespring 33. It will be seen, therefore, that the valve 27 will beeffective to control the degree of fluid pressure at the outlet 26within fine limits.

From the outlet port 26 of the valve 27, fluid under pressure can flowthrough the choke 25 of the inlet port 11 through the second conduit 9into the first conduit 5. From the first conduit 5, the fluid underpressure has two paths open to it. It can flow from the first conduit 5through the second conduit 8 to the outlet port 10 and it can flow fromthe first conduit 5 through the exhaust port 7 past the throttlingelement 13. How much of the fluid in the conduit 5 will flow through theexhaust port 7 will depend upon to what degree the passage through theexhaust port 7 is throttled by the throttling element 13. Moreover, thepressure of the fluid at the outlet port 10 will depend upon at whatrate fluid is exhausted through the exhaust port 7.

The rate at which fluid is exhausted through the exhaust port 7 will bedependent upon the balancing of the forces acting on the element 13.

Considering, firstly, the case when the coil 18 is deenergized, theforces acting downwardly on the element 13 will be, flrstly, the weightof the element 13 and, secondly, the weight of the coil 18, the former17, the button 15, and a part of the weight of the leaf-spring 14.However, from this second force must be subtracted the restoring effortof the leaf-spring l4 tending to counteract the downward deflection ofthe leafspring 14 at its free end due to the weight of the coil 18, theformer 17, the button 15 and that part of the weight of the leaf-spring14. Acting upwardly in opposition to this summation of forces will bethe force exerted on the element 13 by the pressure of the fluid passingthrough the port 7.

With the coil 18 deenergized, the member 4 can be set at an axialposition in the bore 3 to provide an escape of fluid through the port 7which is of such a rate that the pressure of fluid at the outlet port 10is at the required minimum.

If, however, the coil 18 is now energized a further additional force(proportional to the degree of energization of the coil 18) will beexerted downwardly on the element 13 due to the reaction between theelectromagnetic effect resultant upon the energization of the coil 18,and the effect of the permanent magnet 21. This additional downwardforce will tend to move the element 13 downwardly further to throttlethe port 7. This increase in throttling of the port 7 will, on the onehand, reduce the rate at which fluid is exhausted through the port 7and, on the other hand, increase the pressure of this exhausting fluid.Increased throttling of the port 7 will continue until once again, thereis a balance of forces on the element 13 and this increased throttlingof the port 7 will increase the pressure at the outlet port 10.

By suitable choice of proportions between the stitfness of theleaf-spring 14, the diameter of the ball-element l3 and the diameter ofthe exhaust port 7, it is possible to obtain (within very close limits)a linear relationship between the current in the coil 18 and thepressure at the outlet port 10.

Theoretically, however much the clearance between the element 13 and theport 7 is increased, the pressure in the outlet port 10 can never bezero. To avoid the necessity of having a large clearance (and consequentpossible deviation from linearity of the pressure at the outlet port 10with respect to the current flowing in the coil 18) it is desirable thatthe minimum pressure at the outlet port 10 should be about 3 lbs. persq. inch. As has been explained, this minimum pressure is determined bythe axial setting of the member 4 in the bore 3.

Although, as has been stated above, it is theoretically possible toobtain a practically linear relationship between the current flowing inthe coil 18 and the pressure at the outlet port 10, productionvariations between different valves may result in slightly differingcharacteristics between the valves. These differing characteristics mayarise from any one or more of the following:

a. the coil winding 18 may not be identical from one valve to another.

b. the magnetic flux between the poles of the permanent magnet 21 maynot be the same between valves.

c. the diameter of the port 7 may vary within manufacturing tolerances.

d. temperature variations within the coil 18 due either to the fact ofcurrent passing through the coil 18 or a change in ambient temperatureconditions.

Nonlinearity arising from (a), (b) and (c) can be corrected by theaddition of a trimmer resistance across the coil 18 and nonlinearity(due to (d)) above can be corrected by embodying a thermistor in thecoil winding 18.

It will be appreciated that the volume output from the outlet 10 is, ofcourse, very small but if the outlet port 10 is connected to the controlchamber of a relay valve, a large output can be obtained from the relayvalve. Conveniently, the relay valve can be made integral with theabove-described control valve.

The means by which the free end of the leaf-spring 14 can be loaded to avarying degree in the above-described embodiment, has been shown aselectromagnetically operated but alternative resilient forms ofoperating these means could be provided. For example, the loading on theleaf-spring 14 could be provided by a hand or foot lever compressing aspring one end of which was engaged by the lever and the other end ofwhich bore against the leaf-spring 14.

In the above-described embodiment, an increase of pressure at the outletport 10 is consequent upon an increase in current in the coil 18.However, conversely, the arrangement may be such that an increase inpressure at the outlet port 10 is consequent upon a fall in current inthe coil 18.

The control valve specifically described and illustrated in FIG. 1 is soarranged that the coil 18 is normally deenergized and that there isnormally through the seat 7, a maximum escape of fluid due to theminimum throttling effect of the throttling element 13, so as to resultin a minimum pressure at the outlet port 10; energization of the coil 18increasing the throttling effect of the throttling element 13 toincrease the pressure at the outlet port 10. Hence,, in the nonnal"condition of the valve there is a maximum" loss of fluid through theseat 7.

To obviate this disadvantage there may be provided in the passage 40 afurther valve by which the presence of fluid-pressure at the inlet port1 1 from a reservoir, is prevented or permitted in accordance with theoperation of the further valve.

The further valve may also be electromagnetically operated and may, inits energized" condition, be arranged to prevent the supply offluid-pressure from a reservoir to the inlet port 11, energization ofthe further valve causing the valve to open to permit fluid-pressure topertain at the inlet port ll. In this case, the control circuits of saidfurther valve and the coil 18 may be so interlocked that operation of acommon operating member to commence energization of the coil 18immediately causes energization of said further valve so that it isthereby operated to permit fluid-pressure to pertain at the inlet port 11 ready for control by the throttling efiect of the throttling member 13in dependence upon the degree of energization of the coil 18.

However, if the control valve is so arranged that minimum pressure atthe outlet port 10 achieved by minimum throttling of the throttlingelement 13 arises from the maximum energization of the coil 18, thecontrol circuit of the electromagnetic further valve may be sointerconnected with the control circuit of the coil 18 that when thecoil 18 was energized to the maximum extent, the further valve is alsooperated to close the communication between a reservoir and the inletport 11 such as thereby to prevent any pressure obtaining at the inletport 11. In this case, of course, commencement to deenergize the coil 18would immediately cause reverse operation of said further valve suchthat it was thereby opened to permit pressure to obtain at the inletport 1 1.

Referring to FIG. 2 of the drawings, in which like references are usedfor like parts of the valve described with reference to and illustratedin FIG. 1 of the drawings cutoff means 50 is shown embodied in themember 4 of the valve. The member 4 has at its upper end (as viewed inthe drawing) a passage 51 which communicates at its upper end with theexhaust port 7 against which is seatable the ball 13. At its lower end,the passage 51 is closable by a passage-closure member 52 of resilientmaterial carried by a piston 53 slidable within the member 4. The piston53 is resiliently loaded by a spring 54 which urges the piston 53upwardly to urge the passage-closure member 52 in a direction to closethe passage 51. As can be seen, the spring 54 extends between the piston53 and a cap 55 which is screwed into the bottom of the member 4 so thatby screwing the cap 55 inwardly and outwardly of the member 4, thedegree of compression of the spring 54 can be adjusted to vary theloading of the piston 53.

Above the piston 53 and inside the member 4 is a chamber 56 whichcommunicates with the inlet port 26 through a bypass conduit 57.

The operation of a valve modified as shown in FIG. 2 is identical tothat of the valve of FIG. 1 so long as the pressure of the fluid supplyto the passage 51 is of the desired value (which is determined by thedegree ofcompression of the spring 54) for, in this case, the pressureof this supply (which will be applied in the chamber 56) will maintainthe piston 53 and the passage-closure member 52 in the position shown inthe drawing, to permit the normal operation of the valve.

However, should the pressure of the fluid supply drop below apredetermined value, then the force this pressure will exert on thepiston 53 will be overcome by the force exerted by the spring 54 whichwill, therefore, move the piston 53 upwardly to move the passage-closuremember 52 into engagement with the lower end of the passage 51 andthereby seal the passage 51 and prevent the flow of air through thevalve seat 7 and thus of any further depletion of the fluid supplypressure.

Turning now to FIG. 3 of the accompanying drawings,

there is here illustrated a valve which is identical to that of FIG. 1(again, like references indicating like parts) save that the valve ofFIG. 3 is provided with supplementary loading means A which (as willhereinafter be described) upon a breakaway of a trailer from a tractorcontrols the braking apparatus on the trailer to secure a brakingeffort, irrespective of the weight of the trailer, which will result inno more than a predetermined degree of braking of the trailer. Accordingto the particular application of the valve, this may be the maximumpressure or a pressure intermediate the minimum and maximum pressuresobtainable at the outlet port by operation of the first mentionedloading means.

The means A comprises an electromagnet B having an armature C which,when current is flowing in the coil D of the electromagnet B (throughwires E and F) hold the armature C against the effort of a spring C,from engagement with the leaf spring 14. The coil D is normallyenergized from the tractor so that the means A is not normallyeffective.

However, in the event of a breakaway of the trailer, not only will coil18 be deenergized but so also will be the coil D. In this event, thespring C will be effective to move the armature C into engagement withthe leaf-spring l4 and thereby to exert through the armature C aresilient loading on the leafspring 14 which, in the same manner as thepreviously described resilient loading effected by the coil 18, will (byvarying the throttling effect of the ball 13 on the exhaust valve seat7) effect a predetermined fluid pressure at the outlet port 10.

By suitable choice of the value of the spring C and the position of themeans A along the length of the leaf-spring 14, the value of the outputpressure at the output port 10 can be chosen to be a desired value. Inaccordance with the present invention, the value so chosen will be suchas to result in a degree of braking which is somewhat less than themaximum degree of braking normally obtainable on the trailer.

Inan alternative or normally energized arrangement of the control valve,in which removal ofa signal to 18 gives rise to maximum throttling,,removal of the energization of D due to a breakaway can be arranged toso load the spring 14 that maximum or less than maximum throttling isproduced by ball 13 to hold the braking effort to a safe value even foran unloaded trailer.

Having thus described our invention what we claim is:

1. A fluid pressure control valve having a restricted fluid flow inletport, an outlet port, an exhaust port providing a valve seat, athrottling element movable towards and away from the valve seat therebyto effect a varying degree of throttling to the passage of fluid throughthe valve seat, a cantilever-moun ed leaf-spring the free end of whichis operatively engageable by the throttling element and loading means bywhich the free end of the leaf-spring can be loaded such that inoperation of the valve, when a fluid pressure of predetermined valuepertains at the inlet port, the value of the fluid-pressure pertainingat the outlet port is dependent upon the rate of escape of fluid throughthe valve seat which rate is in turn determined by the degree ofthrottling effected by the throttling element consequent upon the degreeof loading of the leaf-spring by said means.

2. A valve as claimed in claim 1, wherein the exhaust port is carried bya member which is movable towards and away from the free end of theleaf-spring to effect adjustment of the minimum throttling effected bythe throttling element.

3. A valve as claimed in claim 2, wherein the member has therein a firstconduit one end of which provides the valve seat, and transverse secondconduits each communicating with the first conduit, one of the secondconduits being in communication with the inlet port and the other of thesecond conduits being in communication with the outlet port, the memberbeing axially movable within a bore in a body of the valve to effectsaid adjustment and the member beingsealed with respect to the bore suchas to prevent intercommunication of the three ports except by way of thefirst conduit.

4. A valve as claimed in claim 1, wherein the throttling element isurged by its weight in a direction tending to seat the element on thevalve seat.

5. A valve as claimed in claim 1, wherein the throttling element isconstituted by a ball.

6. A valve as claimed in claim 5, wherein the leaf-spring is providedadjacent its free-end with a cup-shaped button which serves loosely tolocate the element in position.

7. A valve as claimed in claim 1, wherein the loading means iselectromagnetically operable.

8. A valve as claimed in claim 7, wherein the loading means includes apermanent magnet and a cooperating armature coil.

9. A valve as claimed in claim 8, wherein the permanent magnet isfixedly located and the armature coil is carried by the leaf-springadjacent the free-end thereof.

10. A valve as claimed in claim 9, wherein the leaf-spring is providedadjacent its free-end with a cup-shaped button which serves loosely tolocate the element in position, and wherein the coil is annular and ismounted coaxially with respect to the button.

11. A valve as claimed in claim 1, wherein the inlet port includes achoke.

12. A valve as claimed in claim I, wherein the inlet port is arranged tobe supplied with fluid under pressure via a pressure-limiting valve.

13. A valve as claimed in claim 1, wherein the outlet port is connectedto a relay valve the operation of which is controlled by the pressurepertaining at the outlet port.

14. A valve as claimed in claim 1, wherein there is provided in thefluid path to the inlet port a further valve by which supply of fluidthrough the inlet port can be prevented.

15. A valve as claimed in claim 14, wherein the operation of the furthervalve is so connected with the movement of the throttling element, thatimmediately upon movement of the throttling element towards the valveseat the further valve is operated to allow fluid pressure to pertain atthe inlet port.

16. A valve as claimed in claim 15, wherein the loading means iselectromagnetically operable and wherein the further valve iselectromagnetically operable.

17. A valve as claimed in claim 16, wherein upon operation of theloading means, the further valve is operated to allow pressure topertain at the inlet port.

18. A valve as claimed in claim 1, wherein there is provided cutoffmeans by which the supply of fluid to the valve seat is prevented in theevent of the pressure of the fluid which would otherwise be supplied tothe valve seat being below a predetermined value.

19. A valve as claimed in claim 18, wherein the exhaust port I iscarried by a member which is movable towards and away from the free endof the leaf-spring to effect adjustment of the minimum throttlingeffected by the throttling element, and wherein the cutoff means isembodied in the member.

20. A valve as claimed in claim 19, wherein the member has therein apassage leading to the valve seat, passage-closure means by which thepassage may be sealed, and pressure-sensitive means operativelyconnected to the passage-closure means and subjectable to the pressureof the fluid supplied to the valve seat, as aforesaid, the pressuresensitive member being resiliently loaded in a direction to efi'ectclosure of the passage by the passage-closure means.

21. A valve as claimed in claim 1, wherein there is providedsupplementary loading means by which the leaf-spring can be loadedindependently of the first-mentioned means.

22. A valve as claimed in claim 21 wherein the supplementary loadingmeans is resiliently biased to effect resilient loading of theleaf-spring and the supplementary loading means includes overridingmeans by which the resilient bias is overcome normally to prevent thebias being efi'ective.

23. A valve as claimed in claim 22, wherein the overriding means isvariably operative so as, at will, to be operative to different degreesto counteract the resilient bias.

24. A valve as claimed in claim 23, wherein the overriding meanscomprises an electromagnetic device which is energizable to overcome theresilient bias.

25. A fluid pressure device for converting a variable input signal ofelectrical, mechanical, pneumatic or other form into a proportionalfluid pressure output, including: a restricted fluid flow inlet port, anoutlet port, an exhaust port providing a valve seat and a throttlingelement movable towards and away from the valve seat by loading means independence on said input signal, so that, in operation of the devicewith a fluid pressure of predetermined value at the inlet port, thevalue of the fluid pressure at the outlet port is dependent upon therate of escape of fluid through the valve seat, which rate is determinedby the loading of the throttling element to move it with respect to thevalve seat against the exhaust fluid pressure in response to saidsignal, said exhaust port being carried by a member which is movabletowards and away from the throttling element to effect adjustment to theminimum throttling effected by the throttling element.

26. A valve as claimed in claim 25, wherein the member has therein afirst conduit, one end of which provides the valve seat, and transversesecond conduits each communicating with the first conduit, one of thesecond conduits being in communication with the inlet port and the otherof the second conduits being in communication with the outlet port, themember being axially movable with a bore in a body of the valve toeffect said ad ustment and the member being sealed with respect to thebore such as to prevent lntercommunlcation of the three ports except byway of the first conduit.

27. A valve as claimed in claim 25, wherein the throttling element isurged by its weight in a direction tending to seat the element on thevalve seat.

28. A valve as claimed in claim 25, wherein the throttling element isconstituted by a ball.

29. A valve as claimed in claim 28, wherein the loading means isprovided with a cup-shaped button which serves loosely to locate thethrottling element in position.

1. A fluid pressure control valve having a restricted fluid flow inletport, an outlet port, an exhaust port providing a valve seat, athrottling element movable towards and away from the valve seat therebyto effect a varying degree of throttlinG to the passage of fluid throughthe valve seat, a cantilever-mounted leaf-spring the free end of whichis operatively engageable by the throttling element and loading means bywhich the free end of the leaf-spring can be loaded such that inoperation of the valve, when a fluid pressure of predetermined valuepertains at the inlet port, the value of the fluid-pressure pertainingat the outlet port is dependent upon the rate of escape of fluid throughthe valve seat which rate is in turn determined by the degree ofthrottling effected by the throttling element consequent upon the degreeof loading of the leaf-spring by said means.
 2. A valve as claimed inclaim 1, wherein the exhaust port is carried by a member which ismovable towards and away from the free end of the leaf-spring to effectadjustment of the minimum throttling effected by the throttling element.3. A valve as claimed in claim 2, wherein the member has therein a firstconduit one end of which provides the valve seat, and transverse secondconduits each communicating with the first conduit, one of the secondconduits being in communication with the inlet port and the other of thesecond conduits being in communication with the outlet port, the memberbeing axially movable within a bore in a body of the valve to effectsaid adjustment and the member being sealed with respect to the boresuch as to prevent intercommunication of the three ports except by wayof the first conduit.
 4. A valve as claimed in claim 1, wherein thethrottling element is urged by its weight in a direction tending to seatthe element on the valve seat.
 5. A valve as claimed in claim 1, whereinthe throttling element is constituted by a ball.
 6. A valve as claimedin claim 5, wherein the leaf-spring is provided adjacent its free-endwith a cup-shaped button which serves loosely to locate the element inposition.
 7. A valve as claimed in claim 1, wherein the loading means iselectromagnetically operable.
 8. A valve as claimed in claim 7, whereinthe loading means includes a permanent magnet and a cooperating armaturecoil.
 9. A valve as claimed in claim 8, wherein the permanent magnet isfixedly located and the armature coil is carried by the leaf-springadjacent the free-end thereof.
 10. A valve as claimed in claim 9,wherein the leaf-spring is provided adjacent its free-end with acup-shaped button which serves loosely to locate the element inposition, and wherein the coil is annular and is mounted coaxially withrespect to the button.
 11. A valve as claimed in claim 1, wherein theinlet port includes a choke.
 12. A valve as claimed in claim 1, whereinthe inlet port is arranged to be supplied with fluid under pressure viaa pressure-limiting valve.
 13. A valve as claimed in claim 1, whereinthe outlet port is connected to a relay valve the operation of which iscontrolled by the pressure pertaining at the outlet port.
 14. A valve asclaimed in claim 1, wherein there is provided in the fluid path to theinlet port a further valve by which supply of fluid through the inletport can be prevented.
 15. A valve as claimed in claim 14, wherein theoperation of the further valve is so connected with the movement of thethrottling element, that immediately upon movement of the throttlingelement towards the valve seat the further valve is operated to allowfluid pressure to pertain at the inlet port.
 16. A valve as claimed inclaim 15, wherein the loading means is electromagnetically operable andwherein the further valve is electromagnetically operable.
 17. A valveas claimed in claim 16, wherein upon operation of the loading means, thefurther valve is operated to allow pressure to pertain at the inletport.
 18. A valve as claimed in claim 1, wherein there is providedcutoff means by which the supply of fluid to the valve seat is preventedin the event of the pressure of the fluid which would otherwise besupplied to the valve seat being below a predetermined value.
 19. Avalve as claimeD in claim 18, wherein the exhaust port is carried by amember which is movable towards and away from the free end of theleaf-spring to effect adjustment of the minimum throttling effected bythe throttling element, and wherein the cutoff means is embodied in themember.
 20. A valve as claimed in claim 19, wherein the member hastherein a passage leading to the valve seat, passage-closure means bywhich the passage may be sealed, and pressure-sensitive meansoperatively connected to the passage-closure means and subjectable tothe pressure of the fluid supplied to the valve seat, as aforesaid, thepressure sensitive member being resiliently loaded in a direction toeffect closure of the passage by the passage-closure means.
 21. A valveas claimed in claim 1, wherein there is provided supplementary loadingmeans by which the leaf-spring can be loaded independently of thefirst-mentioned means.
 22. A valve as claimed in claim 21 wherein thesupplementary loading means is resiliently biased to effect resilientloading of the leaf-spring and the supplementary loading means includesoverriding means by which the resilient bias is overcome normally toprevent the bias being effective.
 23. A valve as claimed in claim 22,wherein the overriding means is variably operative so as, at will, to beoperative to different degrees to counteract the resilient bias.
 24. Avalve as claimed in claim 23, wherein the overriding means comprises anelectromagnetic device which is energizable to overcome the resilientbias.
 25. A fluid pressure device for converting a variable input signalof electrical, mechanical, pneumatic or other form into a proportionalfluid pressure output, including: a restricted fluid flow inlet port, anoutlet port, an exhaust port providing a valve seat and a throttlingelement movable towards and away from the valve seat by loading means independence on said input signal, so that, in operation of the devicewith a fluid pressure of predetermined value at the inlet port, thevalue of the fluid pressure at the outlet port is dependent upon therate of escape of fluid through the valve seat, which rate is determinedby the loading of the throttling element to move it with respect to thevalve seat against the exhaust fluid pressure in response to saidsignal, said exhaust port being carried by a member which is movabletowards and away from the throttling element to effect adjustment to theminimum throttling effected by the throttling element.
 26. A valve asclaimed in claim 25, wherein the member has therein a first conduit, oneend of which provides the valve seat, and transverse second conduitseach communicating with the first conduit, one of the second conduitsbeing in communication with the inlet port and the other of the secondconduits being in communication with the outlet port, the member beingaxially movable with a bore in a body of the valve to effect saidadjustment and the member being sealed with respect to the bore such asto prevent intercommunication of the three ports except by way of thefirst conduit.
 27. A valve as claimed in claim 25, wherein thethrottling element is urged by its weight in a direction tending to seatthe element on the valve seat.
 28. A valve as claimed in claim 25,wherein the throttling element is constituted by a ball.
 29. A valve asclaimed in claim 28, wherein the loading means is provided with acup-shaped button which serves loosely to locate the throttling elementin position.